The overall objective of this project is to contribute towards an understanding of the genetic mechanism of malignant transformation of cells. Studies conducted included the preparation of cell lines and the transfer of E. coli xanthine-\ guanine-phosphoribosyl-transferase (xgprt) gene to the transformed human cells. Human cell lines, MGH-1 (a bladder carcinoma); A549 (lung carcinoma) and But-11 (chemically transformed human cells) were tested for growth in agar (0.28%). For each of the above cell lines, single cell clones were isolated from agar and multiplied. Cells from independent clonal cell lines thus produced were then injected subcutaneously into nude mice to test for the tumorigenicity of these cell lines. At the same time, tumors formed in nude mice were isolated and grown in culture. Cultured cells from the tumors produced in nude mice, are being used for further experiments. All of these cell lines have been characterized by karyotypic analysis. A selectable marker into the chromosomes of transformed cells was developed for the xgprt transfer. The chromosome carrying the selectable marker was transferred into normal cells by microcell fusion techniques. In these experiments we are using a cloned E. coli gene for xgprt as a selectable marker for integration into the chromosomes of the transformed cells. The E. coli xgprt gene has been cloned in a vector containing sequences from pBR322 and SV40. This plasmid (psv2-gpt) can be amplified in E. coli and also transferred into mammalian cells. The structure and parameters of transfer of psv2-gpt to the mammalian cells has already been described by Mulligan and Berg. Plasmid carrying E. coli xgprt (psv2-gpt) was amplified in E. coli and plasmid DNA was prepared by described methods. The plasmid DNA was transferred into the human transformed cell lines MGH-1 A549 and Hut-11. Independent colonies following gene transfer have been isolated by selection for the transfer of E. coli xgprt. These colonies are presently being multiplied and will be analyzed for the expression of E. coli xgprt by gel electrophoresis. The colonies will be cultured continuously for 20-30 generations to facilitate the integration of the transferred gene into the recipient cell chromosomes.